1. Field of the Invention
The present invention relates to a wave transmission-reception element for use in an ultrasound probe, a method of manufacturing the element and an ultrasound probe incorporating the element to be inserted into, for example, a blood vessel for ultrasound diagnosis.
2. Description of Related Art
Ultrasound diagnosis, particularly ultrasonogram information, is essential in every field of present-day clinical medicine. For example, in examination of the interior of a blood vessel for an anomaly, such as arterial sclerosis, which is a serious disease derived from a clot resulting from accumulation of cholesterol, direct observation from inside a blood vessel can be expected to yield higher-resolution, more effective imaging than can observation from outside a blood vessel. In this case, since a blood vessel is full of blood, imaging cannot be performed by optical means. In such a case, ultrasound imaging is an effective means for visualization. In ultrasound imaging, an ultrasound probe is inserted into a blood vessel so as to enable visualization of the interior of the blood vessel for diagnosis.
In most conventional methods, an ultrasound beam is transmitted in a radial direction of a blood vessel to thereby obtain a two-dimensional image (as disclosed in, for example, U.S. Pat. Nos. 4,917,097 and 5,603,327 and Japanese Patent Application Laid-Open (kokai) No. 152800/1992). From the viewpoint of medical practice, obtaining a three-dimensional image in real time is preferred. According to a proposed method for obtaining a three-dimensional image, a plurality of piezoelectric elements are arranged in a circle at an end of a probe. One of the elements transmits spherical waves forward, and the remaining elements receive reflections of the spherical waves. The elements sequentially take turns transmitting spherical waves to thereby obtain a three-dimensional image.
In order to obtain a three-dimensional image, this method employs a probe capable of transmitting spherical waves forward. A plurality of fine elements formed from a piezoelectric material are arranged at an end of the probe in order to transmit/receive ultrasound waves.
A practically available material for such an element is a piezoelectric polymer, such as PVDF (polyvinylidene fluoride), which permits fine processing. However, from the viewpoint of sensitivity, piezoelectric ceramic, which has a higher electromechanical coupling coefficient, is preferred as material for the element. Thus, piezoelectric ceramic, such as PT (lead titanate) or PZT (lead zirconate titanate), is used as material for the element. An electrode is formed on each of the front and back faces of an annular piezoelectric ceramic piece. The annular piezoelectric ceramic piece is divided into a plurality of divisions by means of a dicing saw to thereby form a plurality of unit vibration elements, which constitute an ultrasound probe.
According to the above-mentioned method, the annular piezoelectric ceramic piece is divided into elements x serving as unit vibration elements. From the viewpoint of manufacture of elements, the method has an advantage in that a piezoelectric ceramic piece of relatively large size may be formed by a conventional method, since unit vibration elements of small size can be formed through cutting of the piezoelectric ceramic piece as mentioned above. However, as shown in FIG. 22, each element x has a complex shape, such as a portion of a sector. Thus, angle xcex8 of beam spread becomes small. Further, a spherical wave cannot be obtained, and a visualization range A becomes distant and narrow. Since the unit vibration elements are of a complex shape, their vibration modes become complicated, causing difficulty in signal processing. In this case, each of the piezoelectric elements may conceivably be formed into a circular shape. However, since the angle of beam spread as measured in a far sound field is reciprocal to the diameter of a sound source, in order to increase the angle of beam spread, a very fine element must be manufactured. A conventional method for manufacturing a piezoelectric ceramic piece encounters great difficulty in manufacturing a fine element applicable to ultrasound diagnosis effected from inside a blood vessel.
An object of the present invention is to provide a wave transmission-reception element for use in an ultrasound probe capable of solving the above problems, a method for manufacturing the wave transmission-reception element, and a probe incorporating the wave transmission-reception element.
The present invention provides a wave transmission-reception element for use in an ultrasound probe, comprising a base member and a plurality of unit vibration elements embedded in the base member. Each of the unit vibration elements comprises a piezoelectric ceramic piece polarized in a direction extending between its front and back faces, a front electrode formed on the front face of the piezoelectric ceramic piece, and a back electrode formed on the back face of the piezoelectric ceramic piece.
The piezoelectric ceramic piece may assume any of various forms, such as a short cylinder or a quadrangular prism. In the case of a form having a noncircular end face, such as a quadrangular prism, partial electrodes formed on the front and back faces may assume circular shapes so as to produce directivity characteristics substantially similar to those of a cylindrical piezoelectric ceramic piece.
The form of an end face of the piezoelectric ceramic piece is not limited to planar, but may be spherical so as to produce an action of a convex or concave lens. Employment of a spherical end face changes directivity accordingly.
The unit vibration elements having the thus-improved angle-of-beamspread characteristics may be arranged, for example, in a circle and may be operated such that one unit vibration element transmits spherical waves forward while the remaining vibration unit elements receive reflections of the spherical waves and such that the unit vibration elements sequentially take turns transmitting spherical waves to thereby create a three-dimensional image.
Preferably, the plurality of unit vibration elements are embedded in the base member such that each of the electrodes is exposed at the front or back face of the base member. Also, a portion of the base member may serve as an acoustic matching portion or a backing portion.
Specifically, the base member in which the plurality of unit vibration elements are embedded is formed of a member capable of matching the acoustic impedance of a medium, such as blood, within which detection is to be performed; the base member thickly covers the front electrodes of the unit vibration elements so as to form a thick cover portion; and the thick cover portion of the base member serves as an acoustic matching portion. This structure does not require employment of an acoustic matching layer in manufacture of an ultrasound probe from the wave transmission-reception element. The base member may be formed of, for example, an epoxy resin.
Preferably, the plurality of unit vibration elements are embedded in the base member capable of blocking transmission of incident sound waves; the base member thickly covers the back electrodes of the unit vibration elements so as to form a thick cover portion; and the thick cover portion of the base member serves as a backing portion. This structure does not require employment of a backing layer in manufacture of an ultrasound probe by use of the wave transmission-reception element. The base member may be, for example, a mixture of a resin material, such as epoxy resin, fluororesin, or silicone resin; aggregate; and a metallic powder, so as to be able to eliminate incident sound waves through conversion to thermal energy.
Preferably, the front or back electrodes are integrated into a common electrode covering all of the exposed faces of the piezoelectric ceramic pieces, and the common electrode serves a grounding electrode. Employment of the common electrode facilitates formation of the electrode. Alternatively, an independent electrode may be formed on each of the front and back faces of the piezoelectric ceramic pieces. A face on which the grounding electrode is formed may serve as a wave transmission-reception face.
In the above structure, only a certain portion of the unit vibration element has piezoelectric properties. The angle of beam spread and other characteristics depend on the form of an end face of the unit vibration element.
A preferred method for manufacturing a wave transmission-reception element for use in an ultrasound probe and having the above structure comprises the steps of:
(1) forming a piezoelectric ceramic material into a sheet, blanking out blanks from the sheet by use of a die, and firing the blanks to thereby yield piezoelectric ceramic pieces, each having front and back faces;
(2) arranging a plurality of piezoelectric ceramic pieces in place in a die, pouring into the die a material for a base member serving as an acoustic matching portion, and hardening the material in order to obtain the base member in which the piezoelectric ceramic pieces are embedded; and
(3) polishing opposite faces of the base member so as to expose the front and back faces of the piezoelectric ceramic pieces, forming electrodes on the exposed faces of the piezoelectric ceramic pieces on one face of the base member, forming electrodes on exposed faces of the piezoelectric ceramic pieces or forming a common electrode on all of the exposed faces of the piezoelectric ceramic pieces on the other face of the base member, and polarizing the piezoelectric ceramic pieces so as to form unit vibration elements.
According to the above method, the unit vibration elements are integrally held in place in the base member. Through polishing of the base member, the wave transmission-reception element is formed to assume a predetermined thickness. The base member is preferably formed of cold-setting epoxy resin. Notably, the base member may be formed of cement.
According to the above method, after the piezoelectric ceramic pieces are embedded in the base member, formation of electrodes and polarization are performed to thereby yield the wave transmission-reception element. However, the above method may be modified such that the piezoelectric ceramic pieces are individually polished and then subjected to formation of electrodes and polarization, followed by embedment in the base member to thereby yield the wave transmission-reception element.
Alternatively, after the piezoelectric ceramic pieces are individually subjected to formation of electrodes and polarization and are then embedded in the base member, the resultant assembly may be polished at its front and back faces and may again be subjected to formation of electrodes, thus yielding the wave transmission-reception element.
Another method for manufacturing a wave transmission-reception element for use in an ultrasound probe and having an acoustic matching portion comprises the steps of:
(1) forming a piezoelectric ceramic material into a sheet, blanking out blanks from the sheet by use of a die, and firing the blanks to thereby yield piezoelectric ceramic pieces, each having front and back faces;
(2) forming electrodes on the front faces of the piezoelectric ceramic pieces so as to form front electrodes, and connecting lead wires to the corresponding front electrodes
(3) arranging a plurality of piezoelectric ceramic pieces in place in a die, pouring into the die a material for a base member serving as an acoustic matching portion in such a manner as to thickly cover the front electrodes connected to the lead wires, thereby forming a thick cover portion serving as the acoustic matching portion, and hardening the material in order to obtain the base member in which the piezoelectric ceramic pieces are embedded and from which the lead wires are led out; and
(4) polishing the back face of the base member so as to expose the back faces of the piezoelectric ceramic pieces, forming electrodes on the back faces of the piezoelectric ceramic pieces, and polarizing the resultant piezoelectric ceramic pieces to thereby yield unit vibration elements.
According to the above method, after the piezoelectric ceramic elements are embedded in the base member, formation of back electrodes and polarization are performed to thereby yield the wave transmission-reception element. However, the above method may be modified such that the piezoelectric ceramic pieces are individually polished and then subjected to formation of electrodes and polarization; lead wires are connected to the corresponding front electrodes of the piezoelectric ceramic pieces; and the resultant piezoelectric ceramic pieces are embedded in the base member while the lead wires are led out, thereby yielding the wave transmission-reception element.
Alternatively, the piezoelectric ceramic pieces are individually subjected to formation of electrodes and polarization; lead wires are connected to the corresponding front electrodes of the piezoelectric ceramic pieces; the resultant piezoelectric ceramic pieces are embedded in the base member while the lead wires are led out; the back face of the resultant assembly is polished; and the back electrodes are again formed to thereby yield the wave transmission-reception element.
A third method for manufacturing a wave transmission-reception element for use in an ultrasound probe and having a backing portion comprises the steps of:
(1) forming a piezoelectric ceramic material into a sheet, blanking out blanks from the sheet by use of a die, and firing the blanks to thereby yield piezoelectric ceramic pieces, each having front and back faces;
(2) forming electrodes on the back faces of the piezoelectric ceramic pieces so as to form back electrodes, and connecting lead wires to the corresponding back electrodes;
(3) arranging a plurality of piezoelectric ceramic pieces in place in a die, pouring into the die a material for a base member serving as a backing material in such a manner as to thickly cover the back electrodes connected to the lead wires, thereby forming a thick cover portion serving as a backing portion, and hardening the material in order to obtain the base member in which the piezoelectric ceramic pieces are embedded and from which the lead wires are led out; and
(4) polishing the front face of the base member so as to expose the front faces of the piezoelectric ceramic pieces, forming electrodes on the front faces of the piezoelectric ceramic pieces, and polarizing the resultant piezoelectric ceramic pieces to thereby yield unit vibration elements.
According to the above method, after the piezoelectric ceramic elements are embedded in the base member, formation of front electrodes and polarization are performed to thereby yield the wave transmission-reception element. However, the above method may be modified such that the piezoelectric ceramic pieces are individually polished and then subjected to formation of electrodes and polarization; lead wires are be connected to the corresponding back electrodes of the piezoelectric ceramic pieces; and the resultant piezoelectric ceramic pieces are embedded in the base member while the lead wires are led out, thereby yielding the wave transmission-reception element.
Alternatively, the piezoelectric ceramic pieces are individually subjected to formation of electrodes and polarization; lead wires are connected to the corresponding back electrodes of the piezoelectric ceramic pieces; the resultant piezoelectric ceramic pieces are embedded in the base member while the lead wires are led out; the front face of the resultant assembly is polished; and the front electrodes are again formed to thereby yield the wave transmission-reception element.
An acoustic matching layer may be bonded to the wave transmission-reception face of the above-described transmission-reception element, while a backing layer may be bonded to the back face of the wave transmission-reception element, thereby yielding an optimum ultrasound probe. In the case of the wave transmission-reception element having an acoustic matching portion, solely a backing layer may be bonded to its back face. In the case of the wave transmission-reception element having a backing portion, solely the acoustic matching layer may be bonded to its front face.
Since the above-described wave transmission-reception element has fine, circular unit vibration elements, an ultrasound probe using the wave transmission-reception element can transmit forward spherical waves, each having a large angle of beam spread. Thus, when used for examination of, for example, the interior of a blood vessel, the ultrasound probe can produce a three-dimensional image of the interior of the blood vessel in real time. In contrast to conventional practice in which a doctor mentally visualizes a three-dimensional image from obtained two-dimensional images, the ultrasound probe of the present invention enables creation of a three-dimensional image, thereby improving accuracy of ultrasound diagnosis.
A through-hole may be formed at the center of the wave transmission-reception element described above so as to serve as a laser beam path. An ultrasound probe using the wave transmission-reception element can locate, for example, a clot in a blood vessel and can destroy it through emission of a laser beam.